Bug 885023 - Convert mFrameMetrics.mZoom to a CSSToScreenScale. r=BenWa

The mFrameMetrics.mZoom was previously a "resolution-independent" zoom,
which meant it had to always be multiplied by the CalculateIntrinsicScale()
value to be useful. This patch converts it to a "resolution-dependent" zoom,
and includes the intrinsic scale multiplier already. This means it needs
to be updated if either the viewport or composition bounds (which determine
the intrinsic scale) change, but there are only a few places where this
happens and it makes the rest of the code cleaner.